同步电动机是一种交流电动机，转子旋转速度与所提供交流电的频率相同。

交流电动机的原理是由交流电在电动机的定子处产生旋转磁场，因此使电动机转子旋转。

在同步电动机的转子有电磁铁或永久磁铁，使用永久磁铁的称为永磁同步电动机。同步电动机的定子所产生的磁场吸引转子磁场的异极，由于定子所产生的磁场是以若干速度旋转，因此转子会随着定子磁场的旋转速度，以相同的速度旋转。

同步电动机的特点是转速固定，不受电源电压的影响。只要电动机的负载低于其最大转矩，转速也不会受负载的影响。由于这种特性，同步电动机只能使用变频器进行调速。

A synchronous electric motor is an AC motor in which, at steady state, the rotation of the shaft is synchronized with the frequency of the supply current; the rotation period is exactly equal to an integral number of AC cycles. Synchronous motors contain multiphase AC electromagnets on the stator of the motor that create a magnetic field which rotates in time with the oscillations of the line current. The rotor with permanent magnets or electromagnets turns in step with the stator field at the same rate and as a result, provides the second synchronized rotating magnet field of any AC motor. A synchronous motor is only considered doubly fed if is supplied with independently excited multiphase AC electromagnets on both the rotor and stator.